It is possible to transmit audiovisual films and programs in digital form via broadcasting networks of the microwave, cable, satellite type, etc., or via telecommunication networks of the DSL (Digital Subscriber Line) or BLR (local radio loop) type or via DAB (Digital Audio Broadcasting), etc. To avoid pirating of works broadcast in this manner, they are frequently encrypted or scrambled with various well-known means.
As concerns the processing of audiovisual streams constituted of objects, U.S. Pat. No. 6,233,356 B1 discloses systems for encoding and decoding video objects of the MPEG-4 type divided into several layers: A base layer and at least one enhancement layer. The base layer contains sufficient information about the video object for decoding a base representation. The layers called “enhancement layers” contain information for enhancing the quality of the video stream by augmenting the resolution in the spatial and temporal areas. The method of encoding and decoding is capable of encoding and decoding a multilayer MPEG-4 stream. However, that method of encoding is not presented as being able to be used for protecting the original video stream nor for scrambling objects of the visual sequences.
U.S. 2001/0053222 A1 discloses a process and system for the protection of video streams encoded with the MPEG-4 norm. The audiovisual stream is composed of several audio and video objects with the scenic composition being described by the BIFS (Binary Format For Scene) format and also contains IPMP information (“Intellectual Property Management and Protection”). Of all the visual objects contained in the scene, only the video objects are protected. It does not disclose protection for the synthetic objects. Also, the description of scenes in the BIFS format is not secured. Protection is effected by a method of encryption with the aid of four keys. The video object encrypted is then multiplexed with the other video objects in a digital stream sent to the user. The first key, specific for each user, is encrypted with a second one, the result is encrypted with a third key and two pieces of unlocking information are generated from this combination that are sent to the user after identification and authorization. This resulting set is also encrypted with a fourth key and the latter set is multiplexed with the other objects in the audiovisual stream. The last key is periodically updated. Once the stream is protected in this manner, the first key and the two pieces of unlocking information are sent in advance to the address/addressee's equipment, that decrypts the protected stream. The video object is reconstituted with the aid of the fourth key regenerated on the addressed equipment from the two pieces of unlocking information, from the first key, sent in advance, and from the fourth key in addition to information contained in the IPMP part of the MPEG-4 stream sent to the user.
However, that system does not permit a resolution of the security problem because the protection of the video stream rests on an encryption system based on digital keys but for which the video stream transmitted to receiving equipment contains, even in encrypted form, all the information initially present in the original video stream. Thus, the video stream received by the client equipment already contains all the information initially present in the non-encrypted video stream, even if they are stored in a form that does not permit their immediate use.
The known protection techniques are frequently based on a system of protection by key(s) where the user is in possession of the entire visual stream at a given moment. Provided with sufficient calculating means, an ill-intentioned user is therefore capable of finding the protection keys and therefore reconstituting the digital stream for the simple reason that the entire visual information is present in the protected stream.